The present invention relates to a stiffness-adjustable rotary joint.
Stiffness-adjustable rotary joints have been developed and used in particular on robots intended to interact with the human beings, such as for instance humanoid robots and exoskeletons. These rotary joints have two degrees of freedom, namely position and stiffness, and require therefore the use of two separate actuators. They also require the use of elastic elements to provide the compliance which is necessary in order to ensure safety. The stiffness-adjustable rotary joints can be classified, depending on the position of the actuators and of the elastic elements, in the following two categories:                rotary joints with an antagonistic arrangement of the two actuators, in which the two actuators are arranged each in series with a non-linear elastic element and in which the position and the stiffness of the joint are controlled by means of the relative movement of the two actuators; and        rotary joints in which one of the actuators is operable to change the position and the other actuator is arranged in series with an elastic element and is operable to change the stiffness through a mechanism allowing to obtain a non-linear stiffness characteristic.        
The main requirements which have to be met in these rotary joints are:                compact sizes and reduced weight;        capability to change the stiffness with the minimum energy consumption possible; and        wide range of change of the stiffness.        
In most of the previous solutions, whether they belong to the first or to the second category indicated above, stiffness is changed by changing the preload of the elastic elements, which however involves a considerable energy consumption.
A stiffness-adjustable rotary joint is disclosed in the paper “An intrinsically safe actuator with the ability to adjust the stiffness” presented on Jun. 16, 2010 during the 7th IARP Workshop on Technical Challenges for Dependable Robots in Human Environments held in Tolosa, as well as in the paper “A novel variable stiffness actuator: minimizing the energy requirements for the stiffness regulation” presented during the 32nd Annual International Conference of the IEEE EMBS held in Buenos Aires from Aug. 31, 2010 to Sep. 4, 2010. The rotary joint disclosed in the above-mentioned papers basically comprises an output member rotatable about a first axis of rotation, a first actuator device (made in particular as an electric motor and reduction gear assembly) arranged to generate a rotary motion about said first axis of rotation, an input member driven by the first actuator device to rotate about the above-mentioned axis of rotation, a pair of antagonistic springs each interposed between the input member and the output member in such a manner that the torque generated by the first actuator device is transmitted from the input member to the output member via these springs, and a second actuator device arranged to change the distance between the springs and said first axis of rotation, and hence the lever arm of the elastic force produced by the springs, in order to change the stiffness of the joint. According to this known rotary joint, therefore, the stiffness is changed by changing not the preload of the springs, but their position. Moreover, the direction of the movement required to change the position of the springs, and hence to change the stiffness of the joint, is perpendicular to the direction of the elastic force generated by the springs, and accordingly the energy consumption required to change the stiffness of the rotary joint would be theoretically equal to the sole energy consumption of the actuator device used to change the position of the springs. Actually, due to the presence of friction and due to the fact that in positions other than that of equilibrium the elastic force generated by the springs has a certain component, though small, along the direction of displacement of the springs, the energy consumption is higher than the theoretical one. It is however significantly lower than that of the rotary joints in which the stiffness is changed by changing the preload of the elastic elements, and therefore the second actuator device controlling the change of stiffness can be much more compact than that of the other stiffness-adjustable rotary joints. A further advantage of this known solution is that it does not require the use of non-linear springs or mechanisms to provide the non-linear stiffness characteristic required to adjust the stiffness.